This is a cell biological study on the developing nervous system. The main goal is to develop an understanding of growth and differentiation mechanisms in growing nerve cells and their neurites. We will investigate (1) the mechanisms of plasmalemmal expansion, (2) the process of maturation and regional differentiation of the plasma membrane, and (3) the mechanism of guidance and recognition during nervous system development. The major methods to be used involve binding studies with a battery of lectins, toxins, and antibodies on cultured neurons, their rapidly growing processes, and synapses they have formed in vitro. The surface labels will be used in ferritin-conjugated form for their localization and quantitation with the electron microscope. The parameters of interest are: Labeling as a function of the maturational state and of the type of nerve fiber; appearance of specific membrane components, and their relative movement, during nerve fiber growth. Furthermore, we plan immunochemical comparison of membranes from the same neuron in different states of differentiation. These studies are combined with radioautography of the appearance of radioactive membrane precursors on the cell surface, and with freeze-fracture analyses on the types and distribution of intramembranous particles in various regions of neuronal plasmalemma.